To assess the possibility that the mitral valve itself may be involved in hypertrophic cardiomyopathy (HC), we studied mitral valves from 94 patients with HC and 45 in normal control subjects. The area of the mitral leaflets were increased in patients with HC compared to controls (12.9 -/+ 3.7 vs. 8.7 +/- 2.0 cm2; p < 0.0001). This increase was due primarily to an increase in anterior leaflet length (2.2 +/- 0.5 for HC vs. 1.8 +/- 0.3 cm for controls; p < 0.001), since circumference did not differ between the 2 groups. Mitral leaflet area was increased (equal to or less than 12.0 cm2) in 55 (58%) of the 94 valves. In 12 of these 55 valves both the anterior and posterior leaflets were enlarged; the other 43 valves had asymmetric or segmental enlargement of either the anterior leaflet (36 patients) or the midportion of posterior leaflet (7 patients). Nine patients had a congenital malformation of the mitral apparatus in which one or both papillary muscles inserted directly into anterior mitral leaflet.